The present invention relates to a developing apparatus usable with an image forming apparatus for forming an image through an electrophotographic type, an electrostatic recording type or the like, more particularly to a developing device usable with an image forming apparatus such as a copying machine, a printer, a facsimile machine or the like.
In an image forming apparatus which forms color images, four color developers (yellow, magenta, cyan and black developers) are ordinarily used. An image forming apparatus forming color images by superimposing different color toner images is known in which the plurality of developing devices are provided for a single image bearing member. In the case that a color image forming apparatus is used also as a monochromatic copying machine, the frequency of use of the black developing device is very high. Therefore, it is particularly effective to use a one component jumping developing method (jumping developing method with one component developer) of the black image development. In consideration of this, two component developing method (developing method with two component developer) is used with nonmagnetic toner for development with color development materials such as magenta, cyan, yellow and the like, whereas the one component jumping developing method is used with one component magnetic developer, as is known.
However, with the image forming apparatus having such a structure, magnetic carrier particles contained in the two component developer may be introduced to the developing device for the single component developer with the image forming operations with the possible result of reduction of the image density due to the one component developer. To avoid such a problem; various attempts have been made.
For example, Japanese Laid-open Patent Application Sho 59 2056 proposes that carrier removal member is provided opposed to an image bearing member at a position upstream of a developing position to remove the magnetic carrier before the magnetic carrier deposited on the image bearing member reaches the development position of the developing device in the next image forming step. However, with this counter-measurement, the magnetic carrier particles on the image bearing member may pass by the carrier removal member, and therefore, the removal of the magnetic carrier particles is not enough.
Japanese Patent Application Publication Hei 4 47825 proposes, as means for removing the magnetic carrier particles which have been undesirably introduced in the developing device.
A proposal has been made as to use of magnetic force as a means for removing the introduced magnetic carrier. For example, Japanese Laid-open Patent Application Hei 9 211989 discloses that magnetic carrier deposited on the developing sleeve of the one component developing device is removed from the developing sleeve using a permanent magnet or the like. Moreover, Japanese Laid-open Patent Application Hei 5 40406 discloses that point of zero magnetic field is provided on the developing sleeve by the magnetic pole structure in the developing sleeve to remove the magnetic carrier. Even if, however, these methods are used, the magnetic carrier once removed may be again caught by the developing sleeve, and the inventors thought that there was room for improvement.
In addition to the foregoing structure, there are various patterns of structures wherein, for example, a one component developing device and a two component developing device are both provided in the rotary developing device. It is difficult to effectively prevent the introduction of the magnetic carrier into the one component developing device by the disposition of the one component developing device and the two component developing device.
As for means for cleaning the image bearing member, there is a cleaning blade to remove the magnetic carrier before the magnetic carrier is introduced into the one component developing device, and the cleaning blade pressure is raised. However, with this method, vibration or turn-over of the cleaning blade may occur, or the edge of the cleaning blade may be damaged or broken.
Accordingly, it is a principal object of the present invention to provide a developing apparatus with image decrease of a density of an image provided by a first developing means due to deposition of the carrier on the image bearing member from a second developing means and introduction of the deposited carrier into the first developing means, is effectively prevented.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.